It is typical for modern communications systems that there are multiple network operators or service providers. For example there may one or more local telephone networks and operators in each city. Similarly, there may be several long distance networks or carriers for making long distance calls between the local telephone networks or for making international calls. A user has normally a subscription to one of the local telephone networks, but is able to select a long distance carrier on a call by call basis, by dialing a predetermined access code or prefix. In the USA, for example, there is a feature called Equal Access which provides a way for the subscriber to select whatever long distance carrier he/she wants to use when making long distance or international calls. Each long distance carrier is uniquely identified by a digital code called the Carrier Identification Code (CIC), according to the North American Numbering Plan (NAMP). The user dials the CIC by means of an other digital code, called Carrier Access Code (CAC). The CAC sequence is of the form 10xxx, where XXX is the 3-digit CIC and 10 is the prefix indicating that CIC will be following. In near future the CIC will be extended to a 4-digit code and the prefix will then be 101, the CAC being then 101XXXX.
In the USA there is under development the Personal Communications System (PCS) 1900 which is a wireless cellular network based on the European digital mobile communication system GSM (Global System for Mobile Communications). When implementing the Equal Access feature in the PCS, a Preferred Carrier Id (PIC) is added to the subscriber profile, i.e to the subscriber data stored in the subscriber data base (HLR) which is normally used. The long distance call is then as a default rule routed to the long distance carrier indicated by the PIC. The PIC will also apply in the case of call forwarding. The forwarded leg of the call will be routed by using the PIC. During location update procedure, for example, the PIC is transferred to the visited subscriber data base (VLR). The PIC is then used when the subscriber make calls while roaming. If the subscriber wants to use other than the preferred carrier, he/she can override the preferred carrier by dialing the CAC before the destination number (B-number).
This so called "casual dialling" may cause problems to the operator of the local network, if the operator should know exactly the total cost of the calls. The operator may, for example, have service packages where it is needed that the long distance carrier be known by the operator. Such service may be, for example, a prepaid subscription, i.e. the subscriber has paid for the usage of the service in advance. Since, normally the charging of the long distance carrier is independent from that of the local operator, the operator should exactly know the long distance carrier and the charging thereof, in order to be able to monitor the usage of the prepaid fund. In many cases, the companies pay the usage of the mobile phones of their employees. If the company has a deal with one of the long distance operators for a special rate, the casual dialling which allows the user to select any of the long distance carriers may cause extra costs to the company. Therefore, there is a need to control the casual dialling and the selection of the long distance carriers for the calls.